Colored lustrous pigments are known in which the lustrous pigment part is a natural pearlescent material or synthetic pearlescent substance (also called nacreous pigments) and the colors are a wide variety of inorganic and organic coloring agents or dyestuffs. The processes used to make colored lustrous pigments in the prior art encountered numerous difficulties among which the following are noteworthy: severe bleeding of the color on filtration of the coated product from suspension; poor adherence of dyestuffs on the surface of the pigment such that the color could be washed off with water; difficulty of retaining luster with increased color intensity; and non-uniform distribution of the dyestuff on the pigment surface.
The foregoing problems are in part described in U.S. Pat. No. 4,084,983. This patent relates to the use of titanium dioxide-coated-mica pigments to produce colors due to interference phenomenon and additional color effects achieved by coating organic dyes on the surface of these pigments. In an attempt to overcome the problems encountered in the art, the dyestuff is chemically bound on the surface of the pigment with the help of a laking reagent. For example, one laking reagent disclosed is aluminum chloride which on hydrolysis produces layers of aluminum hydroxide with which the dye reacts to form the insoluble color lake thereby permitting the dye to deposit on the surface of the pigment. Although this approach particularly addresses the problem of poor adherence of the organic dye on the surface of the pigment, the problem of heavy bleeding of the uncoated dye remains severe. The failure of the previous art to control this problem has been a serious obstacle to developing a suitable process of preparing colored lustrous pigments of superior quality. This failure was the consequence of the inefficiency of the laking process, which resulted from the manner in which the laking was carried out and from the conditions maintained in the coating of the dye on the surface of the pigment.
U.S. Pat. No. 4,968,351 discloses a superior adsorbed dye colored nacreous pigment and method of producing same by contacting an aqueous dispersion of metal oxide coated substrate nacreous pigment with a solubilized dye and a laking reagent. The process of U.S. Pat. No. 4,968,351 is similar to that of the aforementioned U.S. Pat. No. 4,084,983 in that some of the same reagents can be used to produce the same general type of product. However, the manner in which these reagents are used, the conditions maintained during the laking process, and the quality of reagents employed are significantly different, leading to significantly different results. U.S. Pat. No. 4,084,983 employs a two-step process in which aluminum hydroxide, for example, is first deposited on the platy surface of the pigment and thereafter the aluminum hydroxide coated pigment, with or without isolation from the suspension in which it is formed and re-suspension, is reacted with a dye to form the insoluble color lake. This process depends on how well the dye binds to previously precipitated aluminum hydroxide on the pigment surface. Actually, the binding of the dye to form the color lake, by this method, becomes inefficient because of the loss of active sites as a result of the firmly adhering aluminum hydroxide layers already bound on the surface of the pigment. Fortunately, by following the reverse procedure it is possible to improve not only the efficiency of the laking process but also the quality of the product. In U.S. Pat. No. 4,968,351, an aqueous solution of the dye is combined with the nacreous pigment suspension and it has been found that when the pigment particles are allowed to settle out of suspension, they carry the dye molecules with them by physical adsorption such that much of the dye originally in solution is weakly adsorbed to the pigment particles. At this point in time the dye could be easily washed off the pigment surface with water. The laking reagent (e.g. a hydrolyzable aluminum compound) is added and the aluminum hydroxide precipitated on hydrolysis reacts with the already adsorbed dye to form a firmly adhering color lake on the surface of the pigment.
One the most important synthetic nacreous pigments is titanium dioxide-coated mica which is composed of a mica platelet having an adherent crystalline titanium dioxide coating thereon. The color exhibited is a function of the thickness of the coating. The composite pigment has good reflectivity characteristics, high stability to heat and chemical agents and is non-toxic, which makes it suitable for cosmetic application. The titanium dioxide may be in the anatase form as described, e.g., in U.S. Pat. Nos. 3,087,827; 3,087,828; 3,418,146; and, 3,437,515, or in the rutile crystalline form as described in U.S. Pat. No. 4,038,099.
Metal oxides other than titanium dioxide have been used to prepare similar types of nacreous pigments by coating on the mica platelets. Some examples include ferric oxide (U.S. Pat. No. 3,087,829), zirconium dioxide (U.S. Pat. No. 3,087,828), tin dioxide (U.S. Pat. No. 4,040,859) and the like.
The metal oxide coating on the mica substrate has a high refractive index and provides the optical effects, including high luster or reflectivity, coverage, interference reflection color (if the metal oxide coating is sufficiently thick) and absorption color (if the metal oxide contains color material). The mica, on the other hand, has a low refractive index and functions as a carrier or substrate, as well as contributing to some extent to the optical effect of the pigment. Importantly, titanium dioxide is an approved color additive for foods, drugs, and cosmetics. Mica is not approved for food and drugs.
Unsupported crystalline titanium dioxide platelet nacreous pigments are described in U.S. Pat. No. 3,861,946. The unsupported pigment is realized by coating the titanium dioxide on calcium sulfate chips and platelets followed by dissolving away the calcium sulfate substrate.
U.S. Pat. No. 4,192,691 describes the preparation of unsupported metal oxide nacreous pigments by treating a metal oxide-coated mica nacreous pigment with an aqueous acid solution containing hydrofluoric acid and mineral acid until the desired amount of mica has been extracted. The resulting pigment can be used in a wide variety of cosmetic and plastic applications and possesses improved stability, luster, mechanical integrity and other properties.
A number of the patents describe the removal of the substrate from metal oxide-coated substrates and particularly the dark mica varieties such as phlogopite and biotite. U.S. Pat. No. 4,883,539 describes a process in which a metal oxide-coated phlogopite mica pigment is treated with a mineral acid such as sulfuric acid, hydrochloric acid or nitric acid, preferably combined with some oxidizer such as nitric acid or hydrogen peroxide. U.S. Pat. No. 5,076,849 discloses conducting a two-step process in which the acidic extraction is followed by extracting the acid treated pigment with a base. This procedure is preferably applied to dark mica sorts such as phlogopite or biotite which have lower aluminum content and are therefore easier to dissolve rather than light mica sorts such as muscovite which do not favor a selective extraction of cations.
Unsupported metal oxide nacreous pigments and method of making same are further disclosed in U.S. Pat. No. 5,611,851. Such pigments are prepared by subjecting the metal oxide-coated mica nacreous pigment to an extractive dissolution in acid followed by an extractive dissolution in alkali. The metal oxide-coated mica nacreous pigments can be any of those known heretofore such as those prepared by the methods described in any of the foregoing references. Any of the crystalline forms of the mica substrate can be used although the invention has particular applicability to the muscovite form of mica.